In recent decades, lithium ion secondary batteries become one of the main energy sources for communication electronic products, with the advantages of a high specific energy, a high voltage, a small volume, a light weight, no memory, etc. However, in many cases, due to human misuse, the lithium ion secondary batteries easily lead to hidden troubles dangerous to the user safety such as smoking, firing, even explosion, etc., and therefore, such lithium ion secondary batteries of high capacity and high power have not been widely used in the fields, such as automobile power, etc., hithereto. Hence, improvement of the safety of the lithium ion batteries is a key to develop and generalize the application of lithium ion batteries in the fields such as automobile power, etc.
Current lithium ion battery separators, such as polyethylene (PE), polypropylene (PP), etc, all are difficult to ensure integrity at a high temperature, and the problem on thermal runaway caused by the internal short circuit in the battery due to the shrinkage of the battery separators also often occurs in safety tests such as of overheat, overcharge, etc. Hence, the selection for a high-heat resistant battery separator becomes one of keys to solving the safety of the lithium ion batteries.
Polyimide (PI) is an aromatic polymer containing imide rings on main chains, has excellent heat resistance, chemical stability, good mechanical performance and ultrahigh electrical insulation properties, and can be used as special engineering plastics, high performance fibres, selective permeation membranes, high temperature coatings, high temperature composite materials, etc. Hence, polyimide is a material which is very suitable to be used as high temperature-resistant safe battery separators. Previous documents have disclosed some schemes for solving the heat resistance of the battery separators, but the problem is not basically solved due to the reasons, such as insufficient mechanical strength or overlow porosity or overhigh internal resistance, etc.